Brazing consists of joining base metal surfaces by fusing a filler metal having a lower melting point than the subject base metal without appreciable fusion of the base metal surfaces themselves. For brazing, a flux must be applied to the subject base metal surfaces either prior to or simultaneously with the filler metal. A satisfactory brazing flux flows at a temperature somewhat below the melting point of the filler metal; adheres to or wets the base metal surfaces; facilitates the flow and wetting of the filler metal over the subject base metal surfaces generally by reducing the surface tension of the molten filler metal; removes any oxide coating or other adherent foreign matter present on the subject base metal surfaces without appreciably attacking the base metal surfaces; inhibits re-oxidation of the subject base metal surfaces; and is capable of ready displacement by liquid filler metal either leaving no residue or leaving a readily removable relatively inert residue after completion of the brazing.
The problems attendant brazing light metals such as aluminum and aluminum alloys are particularly onerous because of the adherent and resistant oxide film on aluminum surfaces and because of the relatively low melting point of aluminum as a base metal making necessary the use of lower melting filler metals which are unsuitable for use with high-temperature fluxes. Consequently, the principal area of research has been in the area of new fluxes. One of the early solutions to the flux problem in light metal brazing was set forth in U.S. Pat. No. 2,299,168. Variations in fluxes have appeared in U.S. Pat. Nos. 2,507,346 and 2,552,105. However, recent variations in brazing technique described in U.S. Pat. Nos. 3,667,111 and 3,694,899 show that a completely satisfactory solution has not yet been found.
Of particular interest is U.S. Pat. No. 2,403,110 which describes a brazing paste for joining aluminum surfaces. The brazing paste comprises a mixture of a filler metal, a flux and a vehicle. The filler metal is an aluminum base alloy which melts at a temperature lower than the temperature of the aluminum surfaces or aluminum alloy surfaces to be brazed and can be an aluminum-silicon alloy with or without minor additions of other alloying elements such as zinc, cadmium or tin. The filler metal should be in the form of discrete particles, preferably not much larger than will pass a 50 mesh screen and preferably such that no more than 10 to 20 percent will pass through a 200 mesh screen. The flux is described as generally composed of 50 or more precent of alkali metal chlorides with 50 or less percent of alkali metal fluorides, there being no mention as to whether or not any of said components is anhydrous. The only specifically disclosed flux is composed of 20 weight percent lithium chloride, 36 weight percent potassium chloride, 25 weight percent sodium chloride, 6 weight percent strontium chloride, 1 weight percent potassium fluoride and 12 weight percent trisodium aluminum hexafluoride. The vehicle is described as non-aqueous and capable of volatilizing or othrwise leaving the paste at the brazing temperature and is specifically set forth as a glycol or glycol ether which is liquid at 20.degree. C. and which evaporates without carbonizing. About 10 to 20 percent by weight of the total paste is vehicle, and the balance is flux and filler metal in a weight ratio between 5:1 and 1:1, preferably 3:1. The specific brazing paste described is too dry to be a paste as described and claimed herein, and its viscosity cannot be measured with the equipment employed herein. Even when the amount of vehicle in this paste is trebled, the viscosity of the resulting paste is over 1,000,000 centipoises.
Some brazing pastes heretofore found suitable for brazing light metals have had one particular drawback, namely, poor shelflife as determined by flux activity. When some prior art fluxes are wetted by a liquid vehicle or binder, there is a gradual loss in fluxing action apparently by interreaction of the chlorides and fluorides, and wet shelf-life is limited to about one week at most. When filler metal is included with the flux and vehicle is in a pre-mixed or single component brazing paste, shelf life is further reduced.
Another problem in the preparation of brazing pastes for light metals has been finding a suitable paste combination which on standing remains a uniform mixture rather than separating, which is suitably fluid to be ejected from an orifice to the surfaces to be brazed and which is sufficiently non-fluid to stay where put until brazing can take place.
The principal object of this invention is to provide a single component or pre-mixed brazing paste which is suitable for use in brazing aluminum and which has excellent chemical and physical shelf life. Another object of this invention is to provide novel fluxes which are suitable for use in aluminum brazing pastes and which when wet have shelf-lives of at least six months, even in the presence of filler metal. These objects as well as others which are apparent from the following description are satisfied by this invention.